Dispersion, coating slip and absorptive medium

ABSTRACT

Stable, aqueous dispersion containing powders A and B,—wherein powder A is an amorphous silicon dioxide powder having an average particle diameter of 0.05 to 0.7 μm and a BET surface aa of 5 to 50 m 2 /g, and—wherein powder B is a metal oxide or non-metal oxide powder consisting of aggregates of intergrown primary particles and displays a primary particle size of 5 to 50 nm and a BET surface area of 50 to 400 m 2 /g. Coating slip to form an ink-absorptive coating using the dispersion and at least one hydrophilic binder. Absorptive medium using the coating slip and a support.

The invention provides an aqueous dispersion containing a silicondioxide powder and another metal oxide or non-metal oxide powder. Theinvention also provides a coating slip deriving from this dispersion andan ink-absorptive medium.

Surfaces of absorptive supports can be coated with coating slips toimprove their print properties. Of particular importance are for examplethe adsorption, drying times and adhesion of the ink as well as thegloss of the absorptive medium. Particularly for photograph-typematerials gloss and high ink absorption capacity represent substantialfeatures.

The coating slip for producing a glossy absorptive support generallycomprises an aqueous dispersion of pigments, such as hydrated aluminiumhydroxide, aluminium oxide, silicon dioxide (silica), titanium dioxideand a binder, such as e.g. polyvinyl alcohol, the pigments beingincorporated in the form of powders or as a dispersion of powders.

High-gloss coatings can be obtained for example with fine silicaparticles. The often low stability and the high viscosity of thedispersions used for the coating slips are disadvantageous. Thus thedispersion often has to be produced immediately before its conversioninto a coating slip. More highly filled dispersions are difficult toprocess because of the increased viscosity.

The filler content of the coating slip is an important parameter for thequality of the absorptive medium produced with it and for the economicefficiency of the process. If a coating slip has a high filler contentless coating slip is needed to obtain a specific rate of applicationthan is the case with coating slips having a low filler content. Inaddition, less water has to be evaporated in the case of a high fillercontent, which means that drying is faster. The process can therefore beperformed more economically as compared with a coating slip having a lowfiller content.

A high gloss and a good ink absorption capacity can also be achieved byprocessing means if the coating slip is applied by cast coating. Thisprocess is relatively slow and cost-intensive, however.

In DE-A-100 35 054 cationised fine silica particles with a primaryparticle diameter of 50 nm or less are used in an aqueous dispersion toproduce a coating slip that leads to an absorptive medium with highgloss and good ink absorption capacity.

U.S. Pat. No. 6,284,819 describes a coating slip with a specificviscosity that is obtained from an aqueous dispersion of two particlesdiffering in type and size. The first powder type comprises metal oxideparticles such as e.g. silica, cationised silica or aluminium oxide. Thefirst powder type comprises aggregates of smaller primary particleshaving an average primary particle size of less than 100 nm and anaverage aggregate size of 100 to 500 nm. In addition, the averageaggregate diameter of the particles of the second powder type is atleast half the size of the average aggregate diameter of the firstpowder type. The second powder type comprises metal oxides and syntheticpolymers. The ratio by weight of the particles of the first to thesecond powder type is between 9 and 91 wt. %. An absorptive medium withhigh gloss and good ink absorption capacity can be produced with thecoating slip thus defined. The first powder type of particles isintended to be responsible for the absorption of liquid. The smalleraggregates of the second powder type are intended to fill voids. Overallthe packing density of the coating is increased. The substantial featureis that the average aggregate diameter of the particles of the secondpowder type is at least half the size of the average aggregate diameterof the first powder type. As is shown in the embodiment examples, thecoating slip is obtained by adding a binder, such as e.g. polyvinylalcohol, to a physical mixture of two aqueous dispersions, onedispersion containing the particles of the first powder type, onedispersion containing the particles of the second powder type. Allcombinations of metal oxide particles, regardless of their-specificsurface charge, at a given pH of the dispersion are disclosed in U.S.Pat. No. 6,284,819. This can lead to dispersions that are not stable,that rapidly tend to gel and that are therefore only of limitedsuitability for producing a coating slip.

The examples show that there is a high level of interest in coatingslips and in absorptive media produced with them having high gloss, goodink absorption capacity and rapid drying times. Particular importance isgiven to the dispersions that serve as the starting material for thecoating slips.

The object of the invention is therefore to provide a dispersion havinga high filler content and low viscosity that allows a coating slip to beproduced that, when applied to an absorptive support, produces anabsorptive medium displaying high gloss, good ink absorption capacityand good drying performance.

The invention provides a stable, aqueous dispersion containing powders Aand B,

-   -   wherein powder A is an amorphous silicon dioxide powder having        an average particle diameter of 0.05 to 0.7 μm and a BET surface        area of 5 to 50 m²/g, and    -   wherein powder B is a metal oxide or non-metal oxide powder        consisting of aggregates of intergrown primary particles and        displays a primary particle size of 5 to 50 nm and a BET surface        area of 50 to 400 m²/g, and    -   wherein at a given pH of the dispersion, powders A and B display        the same surface charge sign, and wherein powders A and B have a        zeta potential that gives rise to an electrostatic repulsion        between the particles that is greater than the van der Waals        attraction between the powders, and wherein in the dispersion        the average particle diameter of the group A powder is 60 to        166% of the aggregates of the group B powder and    -   wherein the proportion of powder A, relative to the sum of        powders A and B, is at least 5 wt. %.

The primary particles of these powders are understood to be the smallestparticles in high-resolution-TEM images, which are obviously unable tobe broken down any further. Several primary particles can congregate attheir points of contact to form aggregates. These aggregates are eitherimpossible or very difficult to breakdown again using dispersingdevices. Several aggregates can join together loosely to formagglomerates, a process that can be reversed again by suitabledispersion.

Average aggregate diameter is understood to refer to the equivalentsphere diameter, stated as the volume-weighted median value from peakanalysis. For the powders used in the dispersion it is calculated bydynamic light scattering, for example with a Malvern Zetasizer 3000 HSadevice. If the differences in aggregate diameters of powders A and B ina dispersion are between 60 and 166%, a monomodal distribution ismeasured with this method. This means that the average aggregatediameters of powders A and B are measured as being of the same size iftheir diameters differ by between 60% and 166%. If the average aggregatediameters of two powders in a dispersion differ by more than 60% or bymore than 166%, when measured separately, then the light scatteringdisplays a bimodal distribution of the powder mixture. This distributionlies outside the claimed range.

Stable is understood to mean that over a period of at least one monththe dispersion does not settle out and forms no bottom products. Thisalso means that the dispersion can be transported and does not have tobe produced immediately before use.

Aqueous is understood to mean that the main component of the liquidphase is water.

In order to obtain a stable dispersion it is important that theparticles present in the dispersion display the same surface chargesign. Particles having the same surface charge sign will repel oneanother. If the zeta potential is sufficiently high, the repulsive forcecan overcome the van der Waals attraction between the powder particlesand coagulation or sedimentation of the particles is avoided. The zetapotential is a measure of the surface charge of the particles. It is thepotential at the shear level within the electrochemical double layer ofmetal oxide and/or non-metal oxide particles and electrolyte in thedispersion. The zeta potential depends inter alia on the type ofparticle, for example silicon dioxide, cationised silicon dioxide oraluminium oxide. An important parameter in connection with the zetapotential is the isoelectric point (IEP) for a particle. The IEPindicates the pH at which the zeta potential is zero. In aluminium oxideor cationised silicon dioxide the IEP is at a pH of approximately 9 to10, in silicon dioxide it is below pH 3.8.

The charge density at the surface can be influenced by changing theconcentration of the potential-determining ions in the surroundingelectrolyte. In those dispersions in which the particles carry acid orbasic sites at the surface, the charge can be changed by adjusting thepH. The greater the difference between pH and IEP, the more stable thedispersion.

The zeta potential can be determined for example by measuring thecolloid vibration current (CVI) of the dispersion or by determining theelectrophoretic mobility.

In a preferred embodiment the average primary particle diameters ofpowders A and B can differ by a factor of at least 2, in a particularembodiment by a factor of at least 2.5.

In a particular embodiment the average aggregate diameter of powder Bcan be 80 to 125% of the size of powder A or vice versa. The aggregatediameter of powders A and B is particularly preferably of anapproximately equal size.

The total solids content of the dispersion can be varied over broadlimits. The solids content of powders A and B in the dispersionaccording to the invention can advantageously be between 20 and 80 wt.%.

In an advantageous embodiment the viscosity of the dispersion accordingto the invention can be below a value of 1500 mPas at a shear rate of 12s⁻¹ and a temperature of 23° C. Values of below 1000 mPas at a shearrate of 12 s⁻¹ and a temperature of 23° C. can be particularlypreferred.

There is no restriction on the origin of the silicon dioxide of powderA. Thus ground silica gels, for example those sold by Grace under thename-Sylojet® or Syloid®, can be used.

Pyrogenically produced silicon dioxide powder can preferably be used,however.

Pyrogenically within the meaning of the invention is understood to meanthe oxidation of silicon, as described for example in CA2166844. Silicondioxide powder of this type is sold for example by Elkem under the nameMicrosilica®.

Pyrogenically within the meaning of the invention is also understood tomean the hydrolysis of silicon and aluminium compounds or silicon andtitanium compounds in the gas phase in a flame generated by the reactionof hydrogen and oxygen. The pyrogenically produced silicon dioxide ofpowder A can particularly preferably display a BET surface area of 5 to30 m²/g and a dispersion coefficient Z of less than 40, whereby Z=Y/2X,where X=median value of the particle size distribution, Y=range of theparticle size distribution, relative to 10 to 90% of the cumulativeparticle size. Powder A is produced as described in the Japaneselaid-open specification JP2002-003213 of 9 Jan. 2002. The powdersdescribed therein display particles having an almost perfectly sphericalshape.

FIG. 1 shows the frequency of particles sizes (in %) of a powder havinga specific surface area of 10 m²/g (I) and 30 m²/g (II) as a function ofthe particle size (in μm).

The average aggregate size of powder B can preferably assume values ofbetween 50 and 500 nm.

Powder B of the invention comprises the metal and/or non-metal oxidepowders silicon dioxide, aluminium oxide, titanium dioxide, cerium oxideand zirconium oxide. The surfaces of these powders display acid or basicsites.

There is no restriction on the origin of the metal and non-metal oxides.Pyrogenically produced metal and non-metal oxides can preferably be usedfor the dispersion according to the invention. Pyrogenically producedsilicon-dioxide and aluminium oxide are particularly preferred. The BETsurface area of the powders is between 5 and 600 m²/g.

In an advantageous further development of the invention, powder B can bea mixed oxide powder. Powders of at least two oxides from the groupcomprising silicon dioxide, aluminium oxide, titanium dioxide, ceriumoxide or zirconium oxide can be used as mixed oxide powders.

Mixed oxide is understood to mean the intimate mixture of oxide powdersat an atomic level to form mixed oxygen-metal/non-metal bonds, such ase.g. Si—O—Al or Si—O—Ti. In addition, the primary particles can alsodisplay regions in which the oxide powders are present side by side, forexample regions of silicon dioxide adjacent to aluminium oxide.

Pyrogenically produced mixed oxide powders can preferably be used. Herethe precursors of mixed oxides, separately or together, are transferredto a burner and burnt in a flame and the resulting mixed oxide powdersseparated off. The production of these powders is described for examplein EP-A-585 544, DE-A-199 19 635 (both SiO₂—Al₂O₃ mixed oxides) orDE-A-4235996 (SiO₂—TiO₂ mixed oxide).

The invention also comprises doped metal or non-metal oxides produced bythe method described in DE-A-196 50 500. In particular thesilicon-aluminium mixed oxide described in DE-A-198 47 161.

The invention also comprises powders having a metal or non-metal oxideas core, which is entirely or partially sheathed by a different metal ornon-metal oxide. The sheath can be applied in a liquid medium or bymeans of a deposition process from a vaporous precursor of the metal ornon-metal oxide.

Powders A and B can also be used in cationised form. This can beachieved by treating the powder with a cationic polymer that is solublein the dispersion medium. A polymer having a weight-average molecularweight of below 100,000 g/mol can preferably be used. The range between2000 and 50,000 g/mol is particularly preferred.

Cationic polymers can be polymers having at least one quaternaryammonium group, phosphonium group, an acid adduct of a primary,secondary or tertiary amine group, polyethylene imines, polydiallylamines or polyallyl amines, polyvinyl amines, dicyandiamide condensates,dicyandiamide-polyamine co-condensates or polyamide-formaldehydecondensates.

Those deriving from a diallyl ammonium compound can be preferable,particularly preferably those deriving from a dialkyl diallyl compound,which can be obtained by a radical cyclisation reaction of diallyl aminecompounds and display the structure 1 or 2. Structures 3 and 4 representcopolymers deriving from dialkyl diallyl compounds.

R₁ and R₂ represent a hydrogen atom, an alkyl group having 1 to 4 Catoms, methyl, an ethyl, an n-propyl, an iso-propyl, an n-butyl, aniso-butyl or a tert-butyl group, wherein R₁ and R₂ can be the same ordifferent. A hydrogen atom from the alkyl group can also be substitutedby a hydroxyl group. Y represents a radical-polymerisable monomer unit,such as e.g. sulfonyl, acrylamide, methacrylamide, acrylic acid,methacrylic acid. X⁻ represents an anion.

A poly(diallyl dimethyl ammonium chloride) solution (PDADMAC solution inwater) can be cited by way of example.

The content of cationic polymer can be between 0.1 and 15, preferablybetween 0.5 and 10, particularly preferably between 0.8 and 5 wt. %,relative to the amount of cationic polymer and powder A and/or B.

The dispersion according to the invention can also contain substances toadjust the pH, such as acids, bases or buffer systems, additives tostabilise the dispersion, such as salts, surface-active substances,organic solvents, bactericides and/or fungicides.

The invention also provides a process for producing the dispersionaccording to the invention, which is characterised in that powders A andB are dispersed separately in an aqueous dispersion by means of adispersing device and then combined, or that they are first physicallymixed and then dispersed together, or that they are introduced into thedispersing device in portions and then dispersed together. Apredispersion can optionally take place prior to dispersion.

High-speed mixers or a toothed disc for example are suitable forpredispersion. Rotor-stator machines, such as Ultra Turrax (IKA) orthose manufactured by Ystral, as well as ball mills and attrition mills,are suitable for dispersion. Higher energy inputs are possible with aplanetary kneader/mixer. The efficiency of this system depends on asufficiently high viscosity of the mixture being processed, however, inorder for the high shear energies needed to break down the particles tobe introduced.

Aqueous dispersions having average aggregate diameters of below 100 nmcan be obtained with high-pressure homogenisers. In these devices twopredispersed streams of suspension under high pressure are decompressedthrough a nozzle. The two jets of dispersion hit each other exactly andthe particles grind themselves. In another embodiment the predispersionis likewise placed under high pressure, but the particles collideagainst armoured sections of wall. The operation can be repeated anynumber of times to obtain smaller particle sizes.

The invention also provides a coating slip containing the dispersionaccording to the invention and at least one hydrophilic binder.

Polyvinyl alcohol, partially or entirely saponified, and cationisedpolyvinyl alcohol with a primary, secondary or tertiary-amino group or atertiary ammonium group on the main chain or on the side chain can beused as binder. Combinations of these polyvinyl alcohols with oneanother and polyvinyl pyrrolidones, polyvinyl acetates, silanisedpolyvinyl alcohols, styrene-acrylate latices, styrene-butadiene latices,melamine resins, ethylene-vinyl acetate copolymers, polyurethane resins,synthetic resins such as polymethyl methacrylates, polyester resins (forexample unsaturated polyester resins), polyacrylates, modified starch,casein, gelatine and/or cellulose derivates (for example carboxymethylcellulose) can also be used. Polyvinyl alcohol or cationised polyvinylalcohol can preferably be used.

The coating slip can also additionally contain one or more otherpigments such as calcium carbonates, phyllosilicates, aluminiumsilicates, plastics pigments (for example polystyrene, polyethylene,polypropylene), silicas (for example colloidal silicas, precipitatedsilicas, silica gels, cationised modifications of the cited silicacompounds, aluminium compounds (for example aluminium sols, colloidalaluminium oxides and hydroxyl compounds thereof, such aspseudoboehmites, boehmites, aluminium hydroxide), magnesium oxide, zincoxide, zirconium oxide, magnesium carbonates, kaolin, clay, talc,calcium sulfate, zinc carbonate, satin white, lithopones, zeolites.

The coating slip can display a content of powder A and powder B of intotal 10 to 60 wt. %. It can preferably be greater than 15 wt. %,particularly preferably greater than 25 wt. %.

The coating slip can also contain a proportion of binder, relative tothe sum of powders A and B, of between 3 and 150 wt. %. It canpreferably be between 10 and 40 wt. %, particularly preferably between 3and 15 wt. %.

Crosslinking agents such as zirconium oxides, boric acid, melamineresins, glyoxal and isocyanates and other molecules which link togetherthe molecule chains of the binder system can be used to increase thewater resistance of the binder system and hence of the coating.

In addition, auxiliary substances such as optical brighteners, defoamingagents, wetting agents, pH buffers, UV absorbers and viscosity aids canbe used.

The invention also provides the production of the coating slip, which ischaracterised in that the dispersion according to the invention is addedwith stirring to an aqueous solution of the hydrophilic binder, to whichadditional additives can optionally also be added, and optionallydiluted until the desired ratio of powder and binder and the desiredtotal solids content is established.

The addition sequence is not substantial. Stirring is optionallycontinued for a defined period of time and deaeration is then performedin vacuo if required. Additives are understood to be e.g. pigment,crosslinking agents, optical brighteners, defoaming agents, wettingagents, pH buffers, UV absorbers and viscosity aids.

The invention also provides an ink-absorptive coating using the coatingslip according to the invention and a support. Examples of supports thatcan be used are paper, coated paper, resin films, such as a polyesterresin, including polyethylene terephthalate, polyethylene naphthalate, adiacetate resin, a triacetate resin, an acrylic resin, a polycarbonateresin, a polyvinyl chloride, a polyimide resin, cellophane, celluloidand a glass plate.

So-called photographic base papers, i.e. papers to which one/or morelayers of polyethylene film have been applied to the front and or back,are preferred. Also polyester film, PVC film or precoated papers.

The absorptive medium according to the invention also includes media inwhich the ink-absorptive coating consists of several coating layers ofthe same type or other layers. The coating slip according to theinvention can be found in just one or in several layers. Thus forexample additional ink-absorptive coatings, such as films containingprecipitated silica, can also be applied below the coating slipaccording to the invention. One or more polymer layers (for examplepolyethylene) can also be applied to the substrate and/or to the coatingaccording to the invention, in order to increase the mechanicalstability and/or the gloss in the coating (e.g. photographic base paper,lamination).

The supports can be transparent or opaque. There is no limit to thethickness of the support, but thicknesses of between 50 and 250 μm arepreferably used.

The invention also provides the production of an absorptive medium whichis characterised in that the coating slip is applied to the support anddried. The coating slip can be applied by all conventional applicationprocesses, such as roll coating, blade coating, airbrush, doctor blade(profiled, smooth, slotted), cast coating, film press, size press,curtain coating and slot die application (e.g. casting blade) andcombinations thereof. Processes that allow a very homogeneous coating,such as e.g. cast coating, curtain coating and slot die application, arepreferably used.

The coated substrate can be dried by all conventional methods, such asair or convection drying (e.g. hot air passage), contact or conductiondrying, energy radiation drying (for example infrared and microwave).

It is surprising that the dispersions according to the invention displaya high filler content with low viscosity and that the coating slipsproduced with them display a high gloss. In U.S. Pat. No. 6,284,819 acoating slip is obtained from an aqueous dispersion containing twopowder types of aggregates, the aggregate diameters of the first powdertype being at least 50% smaller than those of the second powder type.The aggregate diameters of the second powder type are preferablysubstantially even smaller, for example below 20 nm. The powder typewith the larger aggregate diameters is intended to be responsible forthe absorption of liquid, the smaller aggregate diameters of the secondpowder type are intended to fill voids. Overall the packing density ofthe coating is increased.

On the other hand, in the dispersion and coating slip according to theinvention the differences in the aggregate diameters of the individualpowder types, in contrast to U.S. Pat. No. 6,284,819, must be no lessthan 60% of the larger aggregates. It is particularly preferable for thediameter of the aggregates to be the same.

An explanation of the very good properties of the dispersion and coatingslip according to the invention cannot be provided at present. FIGS. 2and 3 provide a possible interpretation.

FIG. 2 shows an arrangement of two aggregates having primary particlesof different sizes in a dispersion. The aggregates with the lower BETsurface area have a diameter that is half the size of that of theaggregates with the smaller BET surface area. FIG. 2 corresponds to thefacts described in U.S. Pat. No. 6,284,819. FIG. 2 clearly shows thehigh filler content of the dispersion, which has a negative influence onthe pore volume, however, leading to poorer image properties.

FIG. 3 shows the situation in the dispersion according to the inventionwith two types of aggregates, wherein both types have the same aggregatesize albeit with different primary particle sizes. Large pores areformed with a high filler content.

EXAMPLES

Analytical methods: The viscosity of the dispersions is determined withan MCR300 device with measuring system CC27 from Parr-Physica, withmeasurements taken at shear rates of between 0.01 and 100 s⁻¹. Theviscosity is given at 1 s⁻¹ and 100 s⁻¹ and at 23° C. The viscosity ofthe coating slips is measured with a Brookfield RVT rotary viscometer at100 s⁻¹ and 23° C.

The zeta potential is determined with a DT-1200 device from DispersionTechnology Inc. using the CVI method.

The aggregate size is determined by dynamic light scattering. TheZetasizer 3000 HSa device (Malvern Instruments, UK) was used. Thevolume-weighted median value from peak analysis is given.

The average primary particle sizes of the powders used are determined bytransmission electron microscopy (TEM).

Powders: The pyrogenic silicon dioxide powders OX 10 (BET surface areaapprox. 10 m²/g) and OX 30 (BET surface-area approx. 30 m²/g) fromNippon Aerosil are used as powder A.

The pyrogenic oxide powders DOX 100 (SiO₂ mixed oxide powder with 0.25wt. % Al), AEROSIL® 200 and AEROSIL® 3010 from Degussa AG are used aspowder B.

Dispersions: Analytical data for the dispersions is set out in Table 2.

Demineralised water is used as the dispersion medium for the citedexamples. The demineralised water is measured out and the additiveoptionally dissolved therein. Powder A and then powder B are thenincorporated successively using a high-speed mixer. Dispersion is thenperformed for 30 min on an Ultra-Turrax at 7000 rpm. After approximately24 h the samples are characterised for viscosity, particle size and zetapotential.

Coating Slips

Formulation: An aqueous polyvinyl alcohol solution (PVA Mowiol 40-88,Clariant) with a 12.33% solids content is placed in a beaker and aquantity of water added such that after addition of the dispersion D2 acoating slip is obtained with the desired solids content. The particulardispersion is added to the combination of polyvinyl alcohol solution andwater whilst stirring with a high-speed mixer disc at 500 revolutionsper minute (rpm). Once the addition is completed stirring is continuedfor a further 30 minutes at 500 revolutions per minute. The coatingslips are then deaerated with the aid of a desiccator and a water jetpump.

Coating slip S2-A contains 18 wt. % of dispersion D2, relative to thesolids in the dispersion, and 22 parts of PVA-Mowiol 40-88.

Coating slip S2-B contains 20 wt. % of dispersion D2, relative to thesolids in the dispersion, and 6 parts of PVA Mowiol 40-88.

Index A refers to the coating of films, which is described below, indexB to the coating of paper.

Coating slips S0_(—)2-A and S0_(—)2-B are produced in the same way fromdispersion D0_(—)2 according to the prior art.

The composition, viscosities and pH values of the coating slips arereproduced in Table 3.

Ink-Absorptive Media

The coating slip S2-A is applied with the aid of wet film spiral bladesonto an untreated polyester film (Benn) of thickness 100 micrometers.Drying is performed with a hairdryer. The rate of application obtainedis 25 g/m².

The coated films are printed with an internal test image using an EpsonStylus Color 980 with the settings Photo Quality Glossy Film, 1440 dp,Epson calibration, gamma (D): 1.8.

Coating slip S2-B is applied with the aid of wet film spiral blades ontoa matt inkjet paper (Zweckform, no. 2576). Drying is performed with ahairdryer. The coated paper is then satinised under 10 bar of pressureand at 50° C. with the aid of a laboratory calender. The rate ofapplication of the coating slips S2-B that is obtained is 13 g/m².

The coated papers are printed with an internal test image using an EpsonStylus Color 980 with the settings Premium Glossy Photo Paper, 1440 dpi,bidirectional, Epson calibration, gamma (D): 1.8.

The visual impression of gloss, adhesion and test image for theink-absorptive media produced is reproduced in Table 4. The mediaaccording to the invention M2-A and M2-B display good values for gloss,adhesion and test print. The ink-absorptive media M0_(—)2-A andM0_(—)2-B produced from dispersions D0_(—)2 display good to satisfactoryvalues for gloss, adhesion and test print. In terms of the dryingperformance of the coating slips, the media M2-A and M2-B according tothe invention are clearly superior to the media M0_(—)2-A and M0_(—)2-Baccording to the prior art. TABLE 1 Batch sizes for production of thedispersions Addi- Demin. Powder A Powder B tive water Qty Qty ^((#))Solids Ex. g Type g Type g g wt. % D0_1 1350 OX30 150 0 15 D0_2 1350AE200 150 0 15 D1 OX30 150 AE200 225 0 25 D2 1010 OX30 150 AE200 30040.3 30 D3 1012 OX10 150 AE200 300 38.4 30 D4 730 OX10 150 DOX110 60020.3 50 D5 1067 OX30 75 AE300 300 57.6 25^((#)) Additive: Polyquat 40U05NV from Katpol, Bitterfeld

TABLE 2 Physical-chemical values for the dispersions Av. Av. BET Zetaaggregate surface poten- size D₅₀ area Viscosity Viscosity tial Ex. nmm²/g 1/s⁻¹ 100/s⁻¹ pH mV D0_1 50 30 n.d. n.d. 4.8 n.d. D0_2 245 200 n.d.n.d. 4.3 n.d. D1 298 132 2450 878 4.4 −3.5 D2 173 143 340 120 2.9 +36 D3165 137 235 87 2.9 +37.5 D4 98 54 385 277 2.8 +34.2 D5 168 246 412 2572.8 +38.2n.d. not determined

TABLE 3 Batch sizes and physical-chemical values for the coating slipsPVA content Solids Viscosity Coating ⁽¹⁾ content ⁽²⁾ slip wt. % wt. % pHmPas S0_2-A 36 13 4.7 724 S0_2-B 14 14 4.6 640 S2-A 22 18 3.4 554 S2-B 620 3.2 618⁽¹⁾ Relative to 100 parts of solid in the dispersion⁽²⁾ Viscosity according to Brookfield at 100 rpm in mpas

TABLE 4 Visual impression of gloss, adhesion and test image ⁽¹⁾ for theink-absorptive media Ink- Coating Coating slip absorptive slip Testdrying medium Gloss adhesion print performance M0_2-A + 0 +/0 − M0_2-B+/0 +/0 +/0 − M2-A + + + +/0 M2-B +/0 + + +⁽¹⁾ Each test impression assessed by 3 independent people: ++: verygood, +: good, +/0: good to satisfactory, 0: satisfactory; −: poor

1. A stable, aqueous dispersion, comprising powders A and B; whereinpowder A is an amorphous silicon dioxide powder having an averageparticle diameter of 0.05 to 0.7 μm and a BET surface area of 5 to 50m²/g an wherein powder B is a metal oxide or non-metal oxide powderconsisting of aggregates of intergrown primary particles and whereinpowder B displays a primary particle size of 5 to 50 nm and a BETsurface area of 50 to 400 m²/g; and wherein at a given pH of thedispersion, powders A and B display the same surface charge sign, andwherein powders A and B have a zeta potential that gives rise to anelectrostatic repulsion between the particles that is greater than thevan der Waals attraction between the powders, and wherein in thedispersion the average particle diameter of the group A powder is 60 to166% of the aggregates of the group B powder; and wherein the proportionof powder A, relative to the sum of powders A and B, is at least 5 wt.%.
 2. The dispersion of claim 1, wherein the content of powders A and Bin the dispersion is between 20 and 80 wt. %, relative to the totalamount of dispersion.
 3. The dispersion of claim 1, wherein theviscosity of the dispersion does not exceed a value of 1500 mPas at ashear rate of 12 s⁻¹ and a temperature of 23° C.
 4. The dispersion ofclaim 1, wherein powder A is a pyrogenically produced silicon dioxide.5. The dispersion of claim 4, wherein powder A displays a BET surfacearea of 5 to 30 m²/g and a dispersion coefficient Z of less than 40; andwhereby Z=Y/2X, wherein X is the median value of the particle sizedistribution, and Y is the range of the particle size distribution,relative to 10 to 90% of the cumulative particle size.
 6. The dispersionof claim 1, wherein the average aggregate size of powder B is 50 to 500nm.
 7. The dispersion of claim 6, wherein powder B is a pyrogenicallyproduced silicon dioxide.
 8. The dispersion of claim 6, wherein powder Bis a pyrogenically produced mixed oxide.
 9. The dispersion of claim 8,wherein the mixed oxide is a silicon-aluminium mixed oxide.
 10. Thedispersion of claim 1, wherein powders A and B are in cationised form.11. The dispersion of claim 1, further comprising substances to adjustthe pH and additives to stabilize the dispersion. 12-18. (canceled) 19.The dispersion of claim 11, wherein said substances are acids, bases,buffer systems, or a combination thereof.
 20. The dispersion of claim11, wherein said additives are salts, surface-active substances, organicsolvents, bactericides, fungicides, or mixtures thereof.
 21. A processfor producing the dispersion of claim 1, comprising dispersing powders Aand B in separate aqueous solutions; and combining said solutions.
 22. Aprocess for producing the dispersion of claim 1, comprising mixingpowders A and B; incorporating the mixture thereof into an aqueoussolution; and dispersing the resultant solution.
 23. A process forproducing the dispersion of claim 1, comprising mixing powders A and Bin portions; incorporating the mixture thereof into an aqueous solution;dispersing the resultant solution.
 24. A coating slip to form anink-absorptive coating comprising the dispersion of claim 1 and at leastone hydrophilic binder.
 25. The coating slip of claim 24, wherein thecontent of powder is between 10 and 60 wt. %.
 26. The coating slip ofclaim 24, wherein the content of powder is between 15 wt. % to 60 wt. %.27. The coating slip of claim 24, wherein the content of powder isbetween 25 wt. % to 60 wt. %.
 28. The coating slip of claim 24, whereinthe amount of binder relative to the powders is between 3 and 150 wt. %.29. The coating slip of claim 24, wherein the amount of binder relativeto the powders is between 10 and 40 wt. %.
 30. The coating slip of claim24, wherein the amount of binder relative to the powders is between 3and 15 wt. %.
 31. A process for producing the coating slip of claim 24,comprising adding the dispersion, with stirring, to an aqueous solutionof a hydrophilic binder.
 32. The process of claim 31, further comprisingadding at least one additive to the aqueous solution comprising thehydrophilic binder and the dispersion.
 33. The process of claim 31,further comprising diluting the aqueous solution comprising thehydrophilic binder and the dispersion, until the desired ratio of powderand binder and the desired total solids content is established.
 34. Anabsorptive medium, comprising the coating slip of claim 24 and asupport.
 35. A process for producing the absorptive medium of claim 34,comprising applying the coating slip to the support; and drying theproduct thereof.